The present invention relates to a gas analysis diluter for use with analyzers such as those employed in association with industrial exhaust stacks.
Environmental considerations necessitate the cleaning of exhaust gases which are the by-products of many industrial systems. To determine the effectiveness of the cleansing system, gas analyzers are often incorporated to monitor the efficiency of the gas scrubber and other pollution control equipment. These analyzers are usually of the infrared, chemiluminescent, flame ionization, or flame photometric type wherein the samples are extracted from the source and transported to the analyzer continuously or by a batch process.
The proper functioning of most analyzers necessitates that a suitable interface exist between the analyzer and the source of molecules under analysis. Accordingly, it is necessary to condition the sample to make it acceptable to the analyzer. This is analogous to sample preparation requirements for standard wet chemical or other laboratory techniques. For example, where a flame type detector utilizes the burning of hydrogen and oxygen, the flame cannot be sustained by a sample which does not contain a sufficient concentration of oxygen. Therefore, a flame ionization detector cannot analyze a hydrocarbon stream directly without first supplying air to the analyzer. In most cases a controlled sample stream is mixed with a desired amount of air or oxygen prior to combustion in the detector. The admixture of the sample, and what may be called the transport gases, represents the interfacing of the two gas streams so that the resultant sample is properly conditioned to make it acceptable to the analyzer.
The conditioning of the sample gas entails the removal of particulate matter to very low concentration levels. It is also necessary that water vapor be removed so that the concentration levels thereof are low enough to prevent condensation in the analyzer or the sample transport device, Similarly, the acid vapor or mist concentration must be reduced so as to avoid condensation thereof in the analyzer or the sample transport device. Along with these requirements, there is the necessity of preventing condensation of organic liquids arising from boiler combustion processes.
In present art analyzers the presence of carbon dioxide in the stack exhaust seriously interfers with the operation of chemiluminescent type analyzers. High level concentrations of carbon dioxide can reduce the signal produced by a chemiluminescent reaction for equal concentrations of a particular molecular species from the signal level produced when carbon dioxide is absent from the sample. This negative carbon dioxide interference is commonly called quenching. A common present art technique designed to interface a chemiluminescent analyzer with a source stream involves dilution of the sample by purely pneumatic means. A small sample stream is mixed with a very large stream of clean air or transport gas under controlled conditions. This has the effect of diluting the concentration of all molecular species in the sample stream proportionally to the ratio of the size of the transport gas stream and the sample gas stream. It has been found that there are serious side effects to such a system. The sample is basically unconditioned and this introduces all constituents, in a diluted form, in the exhaust gas stream which include particulates and acid mist into the instrument via the diluter. Such contaminants may plug the equipment or otherwise damage the equipment and thereby introduce faulty readings. The dilution factors utilized are generally insufficient to dilute the sample so as to remove all the harmful particulates or acid mist to insignificant levels. The exposure to contaminants necessitates daily maintenance of the analyzers to keep them in effective operating condition. These systems are not amendable to finer filtration since in practice more extensive filtration interferes with the operation of the system.
It has been found that dilution of sample by means of a diffusion device is more reliable in that the analyzer system is permitted to function at optimum efficiency. The invention recognizes the voids in the prior art for effective sample dilution so that the readings of the analyzer are representative of the exact condition or makeup of the exhaust gas stream.